This invention relates to a method of improving the lap shear stability of a silicone pressure-sensitive adhesive composition produced by a method involving endblocking the residual silanol groups present in the composition with triorganosilyl units derived from triorganosilazanes and/or triorganosilylamines by including an amount of water and an organic compound having both a polar radical and a non-polar portion such as an alcohol in the reaction mixture during the endblocking reaction. This invention also relates to the silicone pressure-sensitive adhesives obtained through the use of that method.
In a U.S. patent application Ser. No. 665,797 entitled "Silicone Pressure-Sensitive Adhesive Process and Product Thereof" which is being filed concurrently herewith and assigned to the same assignee as the present invention (hereby incorporated by reference), John J. Blizzard and Terence J. Swihart teach an improved method of making a silicone pressure-sensitive adhesive by (I) mixing (A) a silanol-containing copolymer resin of 0.6 to 0.9 triorganosiloxy units per SiO.sub.4/2 unit, (B) a polydiorganosiloxane, preferably one containing endblocking X radicals such as silanol or other hydrolyzable radicals, (C) a sufficient amount of an endblocking agent capable of providing endblocking triorganosilyl units such as hexamethyldisilazane or trimethylmethoxysilane to result in a 1:0.8 to 1:3 mole ratio of total moles of silicon-bonded hydroxyl radicals and hydrolyzable endblocking radicals to total moles of triorganosilyl units provided by the endblocking agent, (D) when desirable, a catalytic amount of a mild silanol condensation catalyst if one is not otherwise generated by (C), and (E), when necessary, an effective amount of an organic solvent to reduce the viscosity of the mixture of (A), (B), (C) and (D), and (II) condensing the mixture of (A), (B), (C) and (D) at least until a substantial amount of the endblocking triorganosilyl units have reacted with the silicon-bonded hydroxyl and hydrolyzable X radicals of said (A) and (B), and more preferably, at least until the rate of evolution of condensation by-products is substantially constant and the desired adhesive physical property profile is obtained. Condensation is preferably conducted with heating under solvent reflux conditions. After the condensation is complete, a solvent and/or other ingredients can be added to complete the production of the silicone pressure-sensitive adhesive.
As a result of the simultaneous condensation and endblocking of the resin copolymer and polydiorganosiloxane, the silanol content of the silicone pressure-sensitive adhesive product is reduced to a minimum thereby improving the viscosity stability and physical property stability upon aging of the resulting silicone pressure-sensitive adhesive. A further advantage of the Blizzard and Swihart invention is that the physical properties of the resulting silicone pressure-sensitive adhesive such as tack or adhesion can be modified by changing the level of endblocking agent and/or the type of endblocking agent employed.
As noted in Example 11 of the Blizzard and Swihart Application, the hold time (an accelerated test to measure of lap shear stability which is more fully described, infra) of silicone pressure-sensitive adhesives made using endblocking agents which release ammonia (i.e., organosilazanes) were low relative to other silicone pressure-sensitive adhesive compositions tested. This reduced hold time was thought to be due to the presence of ammonia in the composition and Example 12 showed that it was possible to increase the hold time by adding an amount of water either before or during processing of the pressure-sensitive adhesive composition. The use of water in combination with triorganosilazanes and/or triorganosilylamines to make pressure-sensitive adhesives is claimed in a U.S. patent application Ser. No. 665,805 to John D. Blizzard and Dipak Narula entitled "Silicone Pressure-Sensitive Adhesive Process an Product With Improved Lap Shear Stability-I" which is filed concurrently herewith and assigned to the same assignee as the present invention (hereby incorporated by reference). Compositions produced according to the Blizzard process gave improved hold times over those produced without addition of water, particularly when the compositions contained larger amounts of resin copolymer and thereby contained a higher silicon-bonded hydroxyl content in the composition with which the polar ammonia could associate. Compositions made using alkoxysilanes which are free of ammonia exhibited excellent resistance to creep (high temperature lap shear stability) and would therefore be expected to exhibit improved hold times. Long hold times and resistance to creep are desirable properties where the pressure-sensitive adhesive composition is to be used to adhere an object to a vertical surface or where the composition is used on a pressure-sensitive adhesive tape which will be exposed to stress or heat during use.